Optical detectors are conventionally intended to detect a light intensity and to convert it into an electrical signal, the amplitude of which is proportional to the detected light intensity. These electrical signals are generally processed, particularly in the sense circuit, in order to reproduce an image of the detected scene.
With a view to miniaturizing these detection devices, attempts have been made to integrate the actual detection with the signal processing circuit associated with it.
The principle of an “above IC” structure, moreover, has the advantage of making it possible to achieve a fill factor of close to 100% for the detection pixels, since the detection circuit lies underneath the detection layer.
The detection layer, which is interconnected with the sense circuit through an insulating substrate, traditionally consists of elementary detectors in the form of a matrix. Each of these elementary detectors has a so-called lower electrode which is produced on the said insulating substrate, typically silica or a silicon nitride, and which is connected to the sense circuit by means of an electrical conductor passing through the substrate, traditionally referred to as a “plug” or “via”. This lower electrode is covered with a photosensitive layer, typically consisting of a P-I-N, N-I-P, P-I, N-I, I-P or I-N diode made of amorphous silicon, all of the matrix being covered with a phototransparent upper electrode which is therefore common to all the pixels of the matrix.
The diodes are reverse-biased with a voltage of a few volts between the upper electrode and the lower electrode.
When the intrinsic photosensitive layer absorbs a photon, it therefore emits an electron-hole pair which diffuses towards the metal electrodes, respectively lower and upper, along the field lines imposed by these electrodes, and the particles are collected and stored for a certain integration time, before finally being counted by the sense circuit.
If the photosensitive layer is made of amorphous silicon, the total thickness of the detection layers made of such a material depends partly on the wavelengths to be observed and detected, and typically has a thickness of the order 500 nanometres for correct detection of red light, and 50 nanometres to keep only the absorption of blue light.
The detection layer may be etched at the edge of the matrix, in order to make it possible to make the phototransparent upper layer descend and bias it by providing direct contact with a connector or “plug” of the sense circuit.
Each pixel of the lower electrode is connected individually to the sense circuit, so as to be able to address it, sense it and multiplex the information which is obtained, in order to be able to construct therefrom an image corresponding to the observed detection.
One of the problems often encountered with this type of structure involves the inter-pixel leakage currents at the lower electrodes, in particular due to the appearance of a potential difference between two neighbouring lower electrodes.
In order to overcome this difficulty, U.S. Pat. No. 6,215,164 has proposed a particular structure of P-I-N or N-I-P diodes for photoelectric detectors. They adopt a particular profile, especially with the intrinsic detection layer, obtained by matrixing the doped lower layer so as to cover only the metal of the lower electrode. The photosensitive layer of each of the pixels is isolated from the photosensitive layer of the adjacent pixel by means of an oxide, typically a silicon oxide or Si3N4, or a combination of the two, the purpose of which is to limit the optical intermodulation, a term which is generally used to describe the aforementioned phenomenon of inter-pixel leakage currents.
Matrixing the sensitive layers, however, presents the drawback of demanding regrowth on each level which is intended to be matrixed. It is found that the problems inherent in contamination of the surface and of repeated contact have the effect of increasing the dark current of the detectors obtained in this way, and therefore their sensitivity in respect of weak illumination.
It has also been proposed, for example in document U.S. Pat. No. 6,114,739, to omit the lower doped layer by taking care to correctly select the quality of the metal-photosensitive layer contact of the Schottky contact type.
Another difficulty to be surmounted in the scope of producing such photoelectric detectors is that a high fill factor is intended to be provided in the detector itself. The inter-pixel spacing should consequently be as small as possible, typically of the order of 500 nanometres, whereas the size of the pixels may be as much as a few micrometres.
Reducing the inter-pixel space, however, increases the optical intermodulation phenomena which are precisely that which is intended to be eliminated, or at least limited.
In order to overcome these drawbacks, it has been proposed, for example in document EP-A-0 494 694, to provide the photosensitive layer with a rounded profile at each of the pixels. FIG. 1 accordingly represents a schematic view of the detector in question. It has an insulating substrate (1), in particular made of silicon oxide, and one lower electrode (2) per pixel, surmounted by a photosensitive layer (3) which is made of amorphous silicon and adopts a rounded profile. The assembly obtained in this way is covered with a phototransparent upper electrode (4), which follows the profile of the said photosensitive layer. This type of matrixing, however, also has the drawback of demanding layer regrowth with the same problems of surface contamination as those mentioned in document U.S. Pat. No. 6,215,164.
The technology which is used, namely mounting above an integrated circuit, moreover gives rise to problems in the bonding of the upper layer on the sense circuit. Especially when amorphous silicon detectors are used, the bonding of the thick strained layers with a high hydrogen density constitutes a notable difficulty.
The problem is even more exacerbated when the layer to be bonded is not doped, as is the case for I-P or I-N structures.